The present invention relates to a short fiber-containing polymer composition controlled in volume resistivity which is an index for electrical resistance to a specific range and a method for controlling the volume resistivity of a polymer composition using a short fiber or a short fiber mixture having a specific volume resistivity.
Polymeric composite materials are widely used in various industrial fields for ease of production and in molding thereof. Especially, since polymeric compounds generally show excellent electric insulation, they are often used for electronic parts.
In general, the higher electric insulation is demanded for electronic parts made of polymeric compounds and fiber-reinforced polymeric composite materials, but when they are actually used as a part of the equipments, electrostatic charging occurs resulting in problems such as adsorption of dust. Therefore, the insulation of polymeric compounds which is normally 10.sup.15 -10.sup.16 .OMEGA.cm in terms of volume resistivity must be adjusted to about 10.sup.9 -10.sup.11 .OMEGA.cm in actual use by imparting some electrical conductivity by some means.
One representative method for imparting electrical conductivity to polymeric compounds is to add carbon black and the results of a basic study on this method are reported by Sumita in "Journal of Japan Adhesive Association (Nippon Setchaku Kyokai Shi)", Vol. 23, No. 3, Pages 103-111 (1987).
Although electrical conductivity can be easily imparted by the addition of carbon black, since carbon black has a strong conductivity and is in the form of powder, when dispersibility is increased, it is difficult to enhance stepwise the continuity of electricity and when the addition amount exceeds a certain degree, the electrical conductivity abruptly increases. Therefore, it is very difficult by the use of carbon black to produce polymer compositions or polymer moldings having a low conductivity of about 10.sup.9 -10.sup.11 .OMEGA.cm at a high productivity.
Furthermore, Japanese Patent Kokai (Laid-Open) No. Hei 2-300263 discloses polymeric materials of low surface resistivity made by incorporating superfine carbon fibers into a polymer.
However, though these carbon fibers mentioned in the patent publication are effective for merely antistatic purpose or for removal of problems caused by static electricity, it is difficult by using only the carbon fibers to obtain polymer compositions having a weak electrical conductivity of about 10.sup.9 -10.sup.11 .OMEGA.cm as in the case of carbon black because they are very high in electrical conductivity and besides, it is very difficult to produce industrially the compositions having the specific range of resistivity. Thus, generally use of them cannot be considered.
One attempt to solve the problems which cannot be solved only by using the above-mentioned carbon black or superfine carbon fibers is reported in Japanese Patent Kokai (Laid-Open) No. Sho 63-22861. This patent publication discloses polymer compositions in which char carbon is used as a filler having a selected surface resistivity.
However, since raw materials for the char carbon are straws of natural plants, they are inferior in quality stability and variation is apt to occur in properties of the resulting char carbon. Moreover, there occur changes in availability of the raw material straw and price of the raw material depending on whether the raw material plants are good or bad in harvest and hence, they are unsuitable as industrial materials. Furthermore, according to the Examples given in the above patent publication, even if the char carbon is added, since other filler is carbon black, volume resistivity of the composition is not stable. In addition, the char carbon is in amorphous or particulate form and does not have severe fibrous form. Thus, delicate control of resistivity in the range of 10.sup.7 -10.sup.11 .OMEGA.cm is difficult.